Acoustic pulse echo investigations of earth formations have been described in the art. For example, U.S. Pat. No. 4,255,798 to R. M. Havira describes a method and apparatus for investigating a casing to cement bond by directing acoustic pulses from inside the borehole at radial segments of the casing and analyzing acoustic returns. The U.S. Pat. No. 3,883,841 to Norel et al describes a pad mounted pulse-echo acoustic transducer for analyzing the casing cement interface. Special intermediate layers are employed to match acoustic impedances.
In the investigation of an earth formation, tools employing various sources of energy may be employed. In some tools the spacing between the segment of the tool on which the investigating energy source is located and the borehole wall affects the investigation and techniques are employed to bring this segment of the tool in close proximity with the wall of the borehole. This may involve use of one or several pads pressed against the borehole wall or by pressing the segment of the tool to one side of the borehole wall with tool mounted bow springs to assure that the stand-off sensitive energy source or transducer is in close proximity to the borehole wall. Notwithstanding use of such techniques, the tool segment may be forced to stand away from the borehole wall by the presence of a mudcake. This stand-off may, therefore, undesirably affect the accuracy or interpretation of the tool's investigation of the earth formation. In some instances, cavities in the borehole wall may appear in front of the tool segment and it is desirable to be able to recognize or at least distinguish such cavities from other formation characteristics as a stand-off type measurement.
Techniques have been proposed to measure mudcake thickness. For example, according to one technique, a caliper is employed to measure the borehole diameter and reductions from the original drilling diameter are interpreted as an indication of the thickness of the mudcake. The caliper's use for a mudcake thickness measurement, however, requires extensive depth shifting to relate the measured mudcake thickness to that actually opposite the tool segment carrying the investigating transducer and as a result may not be sufficiently precise when a high spatial resolution investigation of the borehole wall is being made. This problem is particularly acute when the tool's logging motion is irregular, making precise high resolution depth shifting extremely difficult.
An acoustic pulse echo technique to measure mud cake thickness is described in the U.S. Pat. No. 3,175,639 to Liben. In the latter patent an indication of the acoustic impedance of a flushed zone behind the mudcake is derived from a measurement of the change between an acoustic reflection and the applied acoustic pulse. The acoustic pulse generator described in Liben, however, is sensitive to temperature and pressure changes encountered at borehole depths and since these render mudcake thickness measurements less precise, the temperature as a function of depth is computed or a temperature log is made and the pressure as a function of depth is computed. In one technique described in Liben, an acoustic transducer is spaced at some distance from the surface of the borehole wall requiring that the borehole mud be transversed by the applied acoustic pulse and reflections caused thereby along this distance. This tends to introduce inaccuracies in the mudcake thickness measurement and impair resolution, due to factors such as attenuation and beam spreading. The acoustic transducer could be placed in close proximity to the borehole wall as taught by Liben, but in such case the measurement of relatively thin mudcake layers becomes difficult.
When a formation investigation is affected by the presence of standoff, it is desirable to measure the stand-off over a range of thickness and in a manner sufficient to resolve ambiguites introduced by the presence of stand-off. The known prior art techniques for measuring mudcake thickness are, however, not sufficient to aid in resolving ambiguities or correcting measurements made in a high resolution formation investigation which is sensitive to a standoff condition from the borehole wall.